Make gnutls-verify-error work again with url-retrieve-synchronously

* lisp/url/url-gw.el (url-open-stream): Only use :nowait if
we're doing async connections (bug#26835).

* lisp/url/url-parse.el (url): Add an asynchronous slot.

* lisp/url/url.el (url-asynchronous): New variable.
(url-retrieve-internal): Store the value.
(url-retrieve-synchronously): Bind the variable.

lisp/url/url-gw.el

@@ -24,6 +24,7 @@
 ;;; Code:
 
 (require 'url-vars)
+(require 'url-parse)
 
 ;; Fixme: support SSH explicitly or via a url-gateway-rlogin-program?
 
@@ -245,8 +246,9 @@ overriding the value of `url-gateway-method'."
 			   name buffer host service
 			   :type gw-method
 			   ;; Use non-blocking socket if we can.
-			   :nowait (featurep 'make-network-process
-                                             '(:nowait t))))
+			   :nowait (and (featurep 'make-network-process)
+                                        (url-asynchronous url-current-object)
+                                        '(:nowait t))))
                          (`socks
 			  (socks-open-network-stream name buffer host service))
 			 (`telnet

lisp/url/url-parse.el

@@ -36,7 +36,8 @@
                                      target attributes fullness))
             (:copier nil))
   type user password host portspec filename target attributes fullness
-  silent (use-cookies t))
+  silent (use-cookies t)
+  (asynchronous t))
 
 (defsubst url-port (urlobj)
   "Return the port number for the URL specified by URLOBJ.

lisp/url/url.el

@@ -119,6 +119,8 @@ variable in the original buffer as a forwarding pointer.")
 
 (defvar url-retrieve-number-of-calls 0)
 (autoload 'url-cache-prune-cache "url-cache")
+(defvar url-asynchronous t
+  "Bind to nil before calling `url-retrieve' to signal :nowait connections.")
 
 ;;;###autoload
 (defun url-retrieve (url callback &optional cbargs silent inhibit-cookies)
@@ -190,6 +192,7 @@ URL-encoded before it's used."
   (unless (url-type url)
     (error "Bad url: %s" (url-recreate-url url)))
   (setf (url-silent url) silent)
+  (setf (url-asynchronous url) url-asynchronous)
   (setf (url-use-cookies url) (not inhibit-cookies))
   ;; Once in a while, remove old entries from the URL cache.
   (when (zerop (% url-retrieve-number-of-calls 1000))
@@ -232,6 +235,7 @@ how long to wait for a response before giving up."
 
   (let ((retrieval-done nil)
 	(start-time (current-time))
+        (url-asynchronous nil)
         (asynch-buffer nil))
     (setq asynch-buffer
 	  (url-retrieve url (lambda (&rest ignored)